Method and pattern material for precision investment casting



June 28, 1966 G. D. CHANDLEY ETAL 3,257,693

METHOD AND PATTERN MATERIAL FOR PRECISION INVESTMENT CASTING Filed June 19, 1964 Tgmfp HT E m W. E

.0 /2 0 2;: 2252 BY b/W ATTORNEYS United States Patent.

3,257,693 METHOD AND PATTERN MATERIAL FOR PRECISION INVESTMENT CASTING George D. Chandley and William S. Blazelr, Alliance, Shin, assignors to TRW Inc., a corporation of Ohio Filed June 19, 1964, Ser. No. 376,345 14 flaims. (Cl. 22-464) The present invention relates to improvements in the art of precision investment casting, and specifically to the manufacture of relatively porous, ceramic type shell molds in an improved manner.

The manufacture of investment casting molds from destructible patterns dates back hundreds of years, and is typified by the so-called lost wax process. While even to this day, wax is commonly used as a pattern material for this type of mold making process, in recent times it has been supplanted to some extent by patterns made of plastic materials or frozen mercury. Regardless of the pattern material, each of these investment casting procedures has its disadvantages. Wax and mercury are weak materials mechanically, and distort easily. Mercury is hazardous and expensive to use, in view of the extremely low temperatures which have to be maintained from the time the mercury is frozen to the time it is melted out of the green mold structure. Plastic materials are quite expensive, and to a large extent, not recoverable. The plastic materials suffer from the further disadvantage that they must be injected into molds under high pressures. All of the previously used pattern materials, wax, plastic, and mercury exhibit cavitation, that is, flat surfaces appearing on the patterns are likely to be dished in due to shrinkage of the material during solidification. When wax and plastic patterns are used for large castings, large injection presses are required. Furthermore, metal or strong plastic dies are required for wax, mercury, or plastic patterns because of the injection pressures and/or the temperature requirement which must be met. In view of these difficulties, the need still remains for an improved pattern material for a precision investment casting process. The satisfaction of that need is the principal object of the invention.

Another object of the invention is to provide an improved pattern material which is structurally stable and does not distort easily.

A further object of the invention is to provide a pattern material for precision investment mold making which is substantially completely recoverable without difficulty.

Another object of the invention is to provide an improved pattern material for precision investment casting processes which is self-welding.

Another object of the invention is to provide improved elastic materials for the manufacture of expendable dies.

Still a further object of the invention is to provide a pattern material which can be made to expand, contract, or have no change in volume on solidification, depending upon the composition of the pattern making material.

A further object of the invention is to provide an improved method for making ceramic shell molds through the use of an improved pattern making material.

Yet another object of the invention is to provide an improved method for the manufacture of investment casting molds which can be carried out in substantially shorter periods of time than was heretofore considered necessary.

We have now found that a mixture of selenium and sulfur provides a substantially improved pattern material for precision investment casting processes, par- 3,257,693 Patented June 28, 1966 ice ticularly where the composition contains from 5 to by weight selenium, or more preferably, from 5 to 50% by weight selenium. These alloys of selenium and sulfur have been found to be completely compatible with the normal precision investment mold making process wherein successive layers of a particulate ceramic material are applied about the surface of the pattern to build up a green mold, after which the pattern material is removed and the mold is fired at a temperature sufficient to rigidity it. The use of the selenium-sulfur alloys, however, possesses numerous advantages over wax, plastic, or mercury pattern. The materials melt at convenient temperatures, so that it is not necessary to go to the temperature extremes characteristic of the mercury process. The selenium-sulfur alloys do not exhibit the cavitation characteristic of other pattern making materials. The resulting patterns are very hard, strong, smooth, distortion free, and reflect the finest detail of the die from which they are made. The selenium-sulfur molten material can be made to expand, contract, or have no change in volume upon solidification depending on the percent of selenium in the material. Further-more, elastic patterns can be made from selenium-sulfur mixtures. These harden in time to form a dimensionally accurate pattern which can then be processed. This characteristic is helpful in drawing slight undercuts or backdrafted sections from a die.

The sulfur-selenium material can simply be poured into a die which simplifies die construction and eliminates the need for injection presses for small or large castings. The sulfur-selenium material is completely recoverable by autoclave steaming procedures, and is diificult to contaminate. Cheap, wooden dies can be used for certain types of castings, particularly larger ones thereby substantially reducing the cost. Temporary dies made of certain selenium-sulfur mixtures can be remelted and reused, whereas organic materials used for this purpose cannot be so reused.

A further description of the present invention will be made in conjunction with the attached sheet of drawings which illustrate several preferred embodiments.

In the drawings:

FIGURE 1A is a cross sectional view of a split pattern forming die which can be used in accordance with the present invention;

FIGURE 1B is a view of the die shown in FIGURE 1A during the filling with the pattern making material;

FIGURE 1C is a view of the die filled with the solidified pattern material;

FIGURE 1D is a view of the finished pattern after removal of the die;

FIGURE 1E illustrates the pattern material after the mold forming composition has been applied to it;

FIGURE 2A illustrates a preliminary step in the formation of an expendable die by the use of the new pattern making material;

FIGURE 2B illustrates the assembly of FIGURE 2A after solidification of the selenium-sulfur composition about the replica;

FIGURE 2C illustrates the manner in which the relatively elastic selenium-sulfur composition may be split to remove the replica;

FIGURE 2D is a view in elevation of the mass after the split portions are welded back together; and

FIGURE 2E is a plan view of the structure shown in FIGURE 2D.

As shown in the drawings:

In FIGURE 1A, reference numeral 10 indicates generally a die composed of metal or the like, and having a pattern forming cavity 11 therein of the configuration desired in the pattern to be produced.

As illustrated in FIGURE 18, a molten mixture of selenium and sulfur is then poured into the cavity 11 from a vessel 12. The selenium-sulfur mixture sets up within the matter of minutes to form a solidified pattern 13 as illustrated in FIGURE 1C of the drawings. The two halves of the split die are then separated, to liberate the solid pattern which is then ready for use in the mold making process.

While there are a number of manners for forming a ceramic type shell mold about the pattern 13, We particularly prefer to use that method for making refractory molds which is described in Mellen, DeFasselle, and Webb US. Patent No. 2,932,864 of April 19, 1960. In the process there described, the temperature of the pattern material is held substantially constant from the time the pattern is formed until the pattern is removed from the shell mold. Typically, the temperature of the pattern making process may extend from 70 to 80 F.

Initially, the pattern at room temperature or so is dipped in an aqueous ceramic slurry having a temperature about the same as that of the pattern material to form a re fractory layer of a few mils in thickness. A typical slurry may contain ceramic material such as zirconium oxide, a binder such as colloidal silica, and a thickener and low temperature binder such as methyl cellulose. The initial layer while still wet can then be dusted with small particles (40 to 200 mesh) of a refractory glass composition such as that known as Vycor which is a finely divided high silicon oxide glass containing about 96% silica and a small amount of boric acid together with traces of aluminum, sodium, iron, and arsenic. The dusted wet refractory layer on the pattern can then be suspended on a conveyor and moved through a drying oven having a controlled humidity and temperature, wherein the coated pattern is dried adiabatically. When using wet ceramic coatings over the selenium-sulfur pattern, and using air at a Wet bulb temperature of 75 F., the prime coat can be safely dried by air having a relatively humidity of 45 to 55%.

The steps of dipping, dusting, and adiabatic drying are then repeated using air at progressively lower humidities for succeeding coats. For example, the first two coats can be dried with air having a relative humidity of 45 to 55%. The third and fourth coats can be dried with a relative humidity of 35 to 45%, the fifth and sixth coats with a relative humidity of 25 to 30%, and the last coat with a relative humidity of 15 to 25%.

The first layer is preferably applied to a thickness of 0.005 to 0.020 inch, and the fine refractory particles are dusted onto the wet layer with sufiicient force to embed the particles therein. It is preferred that the dusting procedure used provide a dense uniform cloud of fine particles that strike the wet coating with substantial impact force. The force should not be so great, however, as to break or knock off the wet prime layer from the pattern. This process is repeated until a plurality of integrated layers is obtained, the thickness of the layers each being about 0.005 to 0.020 inch. The consolidated layers of ceramic material are indicated at reference numeral 15 of FIGURE 1E.

After the mold is thus built-up on the pattern material, the selenium-sulfur can be removed by placing the same in a conventional steam autoclave operated at temperatures on the order of 300 to 350 F. The pattern material is removed cleanly after a short time in the autoclave, and then the green mold is ready for firing. Generally, firing temperatures on the order of 1,500 to 1,900 F. are used. The resulting shell molds are hard, smooth, and relatively permeable and measure on the order of A, to inch in thickness.

In addition to the zirconium oxide, other suitable refractory materials can be used such as quartz, alumina, silicon carbide, graphite, aluminum silicate, flint and zirconium silicate. The particle size of the refractory material is generally 011 the order of 100 to .400 mesh and is preferably below 325 mesh. The dusting layer can be significantly larger, as indicated previously.

In addition to colloidal silica, binders such as ethyl silicate and other silicate type binders can be employed.

Many of the selenium-sulfur mixtures are temporarily elastic after solidification and prior to crystallization. This characteristic is used to advantage in manufacture of expendable dies, as illustrated in FIGURES 2A to 2B inclusive. In this technique, a male replica 16 is placed in a box 17 and then the molten selenium-sulfur composition 18 is poured into the box 17. After a short time, the molten material solidifies as a deposit 19, and the male replica 16 enclosed in the solidified mass 19 is then removed from the box 17. As illustrated in FIGURE 2C, the elastic mass is then split longitudinally along a line 21 with a sharp, thin blade whereupon the mass may be peeled off from the replica 16. The elastic mass welds back together on touch to form a non-parted die when the material is crystallized. If a parted die is desired, the parts of the mass split are allowed to crystallize separately.

The use of selenium-sulfur molten mixtures to form elastic patterns has the advantage that the patterns retain the shape of the die into which they are poured, even though they are stretched out of shape because of die undercuts or backdrafts during removal of the pattern from its die.

The following specific example illustrates the manner in which the improved pattern material of the present invention may be used in precision investment casting.

Example A bracket casting was made using an aluminum die into which there was poured a molten mixture of 12% selenium, the balance sulfur, at its liquidus melting point. The pattern set up hard in about 3 minutes. The pattern stripped cleanly from the die and showed a smooth surface without cavitation. The pattern was assembled to a dipping hook and dipped through a series of ceramic slurries, with intermediate adiabatic drying, as described previously. The ceramic shell containing the seleniumsulfur pattern was placed in a conventional steam autoclave and melted out at 337 F.- 5 F. After the melt out, the mold was fired out at 1,875 F., cleaned, preheated to 1,650 P. for 1 /2 hours, and filled with a molten nickel-chromium alloy at 3,100" F. The casting which resulted was of completely acceptable quality and met dimensional requirements.

It should be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

We claim as our invention:

1. The method of making a mold which comprises forming a molten mixture of selenium and sulfur containing at least 5% by Weight selenium, freezing said mixture to form a pattern of an article to be reproduced, applying a ceramic mold making composition to the resulting pattern, and thereafter removing the pattern from the mold thus produced.

2. The method of making a mold which comprises forming a molten mixture of selenium and sulfur containing from 5 to by weight selenium, freezing said mixture to form a pattern of an article to be reproduced, applying a ceramic mold making composition to the resulting pattern, and thereafter removing the pattern from the mold thus produced.

3. The method of making a mold which comprises forming a molten mixture of selenium and sulfur containing from 5 to 50% by weight selenium, freezing said mixture to form a pattern of an article to be reproduced, applying a ceramic mold making composition to the resulting pattern, and thereafter removing the pattern from the mold thus produced.

4. The method of making a mold which comprises forming a molten mixture of selenium and sulfur containing from 5 to 75% by weight selenium, freezing said mixture to form a pattern of an article to be reproduced, applying a ceramic mold making composition to the resulting pattern, steaming out the pattern material from within the resulting mold, and firing the mold at an elevated temperature Sllfl'lCiCIlt to 'rigidify the same.

5. The method of making an expendable pattern die which comprises providing a male replica of the desired pattern, pouring a molten selenium-sulfur mixture about said replica, solidifying said mixture about said replica, splitting the resulting solidified selenium-sulfur mixture before crystallization thereof, peeling the freshly solidified mixture off said replica, and pressing the split portions together to Weld the same together.

6. The method of claim 5 in which said selenium-sulfur mixture contains from 5 to 75 by Weight selenium.

7. The method of claim 5 in which said selenium-sulfur mixture contains from 5 to 50% by weight selenium.

8. In a precision investment mold making process in which an impermanent pattern is coated with successive layers of a ceramic mold forming composition, the pattern is removed, and the mold is rigidified, the use of a mixture of selenium and sulfur containing from 5 to 75 by weight selenium as the pattern material.

9. A coated pattern for the production of refractory molds comprising a pattern composed of a mixture of selenium and sulfur containing at least 5% by weight selenium, said pattern being coated with adherent layers of finely divided refractory particles.

10. A coated pattern for the production of refractory molds comprising a pattern composed of a mixture containing 5 to 75% by weight selenium and the balance sulfur, said pattern being coated with adherent layers of finely divided refractory particles.

11. A coated pattern for the production of refractory molds comprising a pattern composed of a mixture containing from 5 to by weight selenium and the balance sulfur, said pattern being coated with adherent layers of finely divided refractory particles.

12. A method of making a porous ceramic shell mold which comprises providing a pattern containing 5 to References Cited by the Examiner UNITED STATES PATENTS 1,238,789 9/1917 Kralund 22-196 2,204,123 6/1940 Collins 22-196 2,932,864 4/1960 Mellen et a1. 22192 3,148,422 9/1964 Payne 22-196 FOREIGN PATENTS 614,155 2/1961 Canada.

MARCUS U. LYONS, Primary Examiner. 

1. THE METHOD OF MAKING A MOLD WHICH COMPRISES FORMING A MOLTEN MIXTURE OF SELENIUM AND SULFUR CONTAINING AT LEAST 5% BY WEIGHT SELENIUM, FREEZING SAID MIXTURE TO FORM A PATTERN OF AN ARTICLE TO BE REPRODUCED, APPLYING A CERAMIC MOLD MAKING COMPOSITION TO THE RESULTING PATTERN, AND THEREAFTER REMOVING THE PATTERN FROM THE MOLD THUS PRODUCED. 